Metamaterial refers to a material or an electromagnetic structure designed artificially to exhibit a special electromagnetic characteristic which cannot be generally found in the nature. The term Metamaterial as defined herein and in the present art, generally refers to a material or an electromagnetic structure having permittivity and permeability whose values are all negative numbers.
Such material is also referred as to a “double-negative (DNG) material” in terms of having two negative parameters. Metamaterial is also referred to as a “negative refractive index material (NRM)” in terms of having a negative reflection coefficient by negative permittivity and permeability. Metamaterial was originally researched by V. Veselago, a physicist of the Soviet Union in 1967, but after 30 years have passed since that research and application on a concrete implementing method is currently in progress.
Based on such characteristics, the electromagnetic waves inside Metamaterial are transferred by Fleming's left hand rule, but not Fleming's right hand rule. In other words, a phase propagation (phase velocity) direction and an energy propagation (group velocity) direction of the electromagnetic waves are opposite to each other. For this reason, Metamaterial is also referred to a left-handed material (LHM). Also, Metamaterial exhibits a non-linear relationship between β (phase constant) and ω (frequency) as well as a characteristic in which its characteristics curve also exists in a left half plane of a coordinate plane. By virtue of such characteristics, Metamaterial enables implementation of a broad-band circuit due to a small phase difference according to frequencies, as well as enables implementation of a miniature circuit since a phase change is not proportional to the length of a transmission line.
Research on methods of implementing Metamaterial is in progress continuously, and among them, a method of implementing Metamaterial using a left-handed (LH) transmission line is known in the art.
FIG. 1 is a circuit diagram showing an equivalent circuit of a conventional transmission line and an LH transmission line according to the prior art.
As shown in FIG. 1(a), the equivalent circuit of the conventional transmission line is represented by a serial inductor LR and a parallel capacitor CR. On the other hand, as shown in FIG. 1(b), the equivalent circuit of a Metamaterial transmission line, i.e., an LH transmission line is represented by a serial capacitor CL and a parallel inductor LL. It has been known that a Metamaterial having the above-mentioned electromagnetic characteristics can be implemented by realizing such a transmission line.
Such an LH transmission line is implemented with the transmission line as shown in FIG. 2, which is disclosed in U.S. patent application Ser. No. 11/092,143, which issued as U.S. Pat. No. 7,330,090 on Feb. 12, 2008 to Itoh, et. al. The transmission line can be implemented using a known substrate such as an FR4 substrate, etc. Specifically, the transmission line includes a dielectric layer 400, an interdigital capacitor 100 and stub inductors 200 formed by printing, depositing or etching a first conductive element disposed on the top surface of the dielectric layer 400, and a ground plane 300 formed by printing, depositing or etching a second conductive element disposed on the underside of the dielectric layer 400.
[9] The serial capacitor C of the LH transmission line shown in FIG. 1(b) is implemented with the interdigital capacitor 100. The interdigital capacitor 100 is implemented to accomplish miniaturization of a device unlike a conventional multi-layered capacitor formed disposing a plurality of conductive layers on a dielectric layer and to be easily included in the transmission line. The serial capacitor C is configured such that sets of two fingers 110 and 120 are alternately arranged spaced apart from one another to have a capacitance by an electromagnetic coupling between the fingers. Each set of fingers 110 and 120 are electrically connected at one ends thereof to one another so that capacitances between a plurality of fingers, i.e., a capacitance between a finger 110a and a finger 120a and a capacitance between a finger 110b and a finger 120b are synthesized in parallel to have larger capacitance.[10] In the meantime, the parallel inductor L of the LH transmission line is implemented with the stub inductor 200 as a short circuit stub. The stub inductor 200, which is an elongated conductor, is connected at one end thereof to the ground plane 300 through a via hole 210. The stub inductor 200 employing an inherent inductance of a general conductor has an inductance determined depending on its length. Thus, the transmission line which can be represented in FIG. 1(b) is implemented so that a transmission line having a desired length can be implemented through the cascade connection of a plurality of cells using a cell. In this case, capacitance and inductance occurring inevitably in each conductor exhibit an electrical characteristic in which the RH transmission line and the LH transmission line are combined.
The structural limitation of each constituent element of such a conventional LH transmission line contributes to restriction of performance improvement of the transmission line. First the interdigital capacitor 100 is disadvantageous in that its capacitance is smaller than a capacitance of a multi-layered capacitor. The reason for this is that the area of conductors electromagnetically connected opposite to one another is relatively small in the interdigital capacitor 100. Besides this, since it is required that the respective fingers should be formed to accurately intersect with one another in a criss-cross fashion, the interdigital capacitor 100 is very difficult to fabricate and process. The multi-layered capacitor has a demerit in that the adjustment of capacitance is performed only by adjusting the interval between the conductors and the area of the conductors, a degree of freedom of design is degraded, which generally makes it difficult to be used in the LH transmission line. U.S. patent application Ser. No. 10/904,825, which issued as U.S. Pat. No. 7,190,014 on Mar. 13, 2007 to Kao, and U.S. patent application Ser. No. 11/234,276, which published as U.S. Patent Publication No. 2006/0086963 on Apr. 27, 2006 to Wu and now abandoned, disclose a stacked interdigital plate capacitor structure. However, the above U.S. patents entail a problem in that since it has a similar construction as that of the multi-layered capacitor, a degree of freedom of design is decreased and its structure is complicated to thereby increase the manufacturing cost.
Meanwhile, since an inductance of the stub inductor 200 is determined depending on the length of the conductive element, the length of the conductive element must be increased so as to increase the inductance of the stub inductor, which results in an increase of the size of the inductor 200. In addition, in case where the wavelength of a signal is twice as long as the length of the stub inductor 200, the stub inductor 200 is operated as a λ/2 resonator so that a cutoff frequency appears in a frequency response as well as the inductor can be operated only in a length of less than ¼ of the wavelength in terms of an impedance characteristic. Thus, it is impossible to use the stub inductor 200 in a broad frequency band.
Due to the structural limitation of the interdigital capacitor 100 and the stub inductor 200, the conventional LH transmission line has a lot of limitations in expansion of the transmission band and miniaturization thereof.